Incident Summary
On an instrument landing system (ILS) approach to runway 19L at Brisbane, the captain was manually flying while the first officer handled the flight path reference (FPR) line function. The captain requested the first officer to adjust the FPR value to the runway's glideslope angle of 3.0°. To do this, the first officer needed to press the FPR button and then turn the FPA SEL knob. Instead, the first officer inadvertently pressed the FPA button, which was a similar-looking button co-located with the FPR button.
Error Analysis
The first officer's action was identified as a 'slip' type error, defined as an incorrect execution of a correct intention. The FPA and FPR buttons were both used with the same FPA SEL control knob. The first officer's intention was to press FPR, but the close proximity and similar appearance of the buttons led to the slip. The aircraft manufacturer reported no awareness of similar occurrences that would suggest a significant ergonomic issue. The captain had not briefed the use of the FPR line function during the approach briefing, but this was unlikely to have influenced the slip error, as the first officer knew which button to press.
Flight Crew Response
Pressing the FPA button disengaged the ILS approach mode and changed the active flight director modes from glideslope and localiser to flight path angle and roll mode. The flight director then displayed the aircraft's flight path angle at the time of the button press (3.3° nose down), and the flight path reference line turned solid with a readout of 3.3°. The unexpected mode change surprised the crew, diverting their attention from monitoring the flight path. They became preoccupied with correcting the mode and recapturing the ILS flight director modes. As a result, the aircraft descended below the glideslope, and the approach became unstable with respect to glideslope deviation, which neither flight crew member recognized. The descent rate exceeded 1,000 feet per minute for about 6 seconds, though the stabilized approach criteria allowed momentary excursions. The first officer called out 'slope' but did not make 'slope limit' or 'unstable' calls to initiate a go-around. The captain began recovering the aircraft's descent shortly after the glideslope deviation exceeded 1.0 dot.